There is an ever present desire to maximize the speed of digital communications via networks, and particularly telecommunication networks, such as public telephone systems. Accordingly, telecommunication network providers now offer customers many options for coupling to the telephone network in addition to the standard analog based connection commonly referred to as POTS (Plain Old Telephone System). Some of the options that are widely available are Integrated Services Digital Network (ISDN) lines, T-1 lines, E-1 lines, digital subscriber lines (DSL) and asymmetric digital subscriber lines (ADSL).
ADSL's can provide very high data speeds, such as on the order of several megabits per second, over a standard twisted wire pair. Unlike the traditional data modems used for analog communication with a telephone central office via a twisted wire pair, ADSL requires modems both at the subscriber end and at the telephone company Central Office end. Current ADSL systems employ discrete multitone (DMT) technology to implement high bandwidth communications, such as for digital TV broadcast, on demand video, high speed video-based internet access, work at home digital file transfer, teleconferencing, home shopping, and other information services over existing twisted wire pair telephone lines.
Several DMT standards have been promulgated. For instance, the International Telecommunications Union (ITU) has promulgated a standard for ADSL that is commonly termed G.lite and which is set forth in ITU-T specification G.992.2, incorporated herein by reference. Another standard, promulgated by ANSI, is commonly termed Heavy ADSL and is set forth in ANSI specification T1.413, issue 2, also incorporated herein by reference. In DMT communications, data is sent in frames. A frame is comprised of a plurality of samples, each frame including data samples and cyclic prefix samples. Data samples comprise most of the frame and the collection of data samples in a single frame comprise one DMT symbol. The cyclic prefix is added at the beginning of each frame and comprises the last L samples of that frame. Accordingly, the cyclic prefix samples are between the DMT symbols in the data stream. The purpose of the cyclic prefix is to help avoid inter-symbol interference (ISI). The frame and cyclic prefix is of a standardized length. For example, in heavy ADSL, each symbol comprises 512 samples with 256 tones (32 tones for upstream communications), each tone having a real and an imaginary portion. Heavy ADSL utilizes a cyclic prefix of length L=32 samples. Accordingly, a frame has 544 samples.
FIG. 1 is a block diagram of the basic ADSL modem functions, as would be well known to those of skill in the art. The upper half of the diagram represents functions in the transmit direction while the lower half represents functions in the receive direction.
It should be noted that FIG. 1 is a functional block diagram and that the blocks shown therein do not necessarily correspond to separate physical circuits. In fact, most if not all of the functions will be performed by one or more digital processors such as, but not limited to, a digital signal processor, a micro processor, a programmed general purpose computer, etc. It also is possible that part or all of the functions of some of the blocks may be implemented by analog circuitry.
In the transmit direction, digital data is transmitted from the transmitter 102 to a scrambler 104 that scrambles the data for transmission. The data is then processed through a forward error correction (FEC) encoder 106 which adds syndrome bytes to the data. The syndrome bytes will be used for error correction by the receiver at the receiving terminal. Next, as shown in block 108, the transmit data is encoded using quadrature amplitude modulation (QAM). The data is then converted from the frequency domain to the time domain via Inverse Fast Fourier Transform (IFFT) 110.
A 1:4 interpolator 112 interpolates the output of IFFT block 110 to produce 512 samples from the 128 samples output from block 110. The 32 sample cyclic prefix is added to each frame in block 114. The data is then forwarded to a coder/decoder (CODEC) 116. The CODEC encodes the data for transmission over the twisted wire pair to the receiving device.
In the receiver portion of transceiver 100, the received signal is passed from the twisted wire pair through the CODEC 116 where it is decoded. It is then passed to a time domain equalizer (TEQ) 118 to shorten the channel impulse response. Then, in 120, the cyclic prefix is removed. It will be appreciated by those of skill in the related arts that, if the length of the channel impulse response is less than or equal to the cyclic prefix length, then the Inter-Symbol Interference (ISI) can be eliminated by removing the Cyclic Prefix (CP) length. Furthermore it is possible to compensate for channel distortion with the frequency domain equalization (FEQ) in block 124, discussed further below.
An echo canceller (EC) 134 is coupled between the transmit path and the receive path and creates an echo cancellation signal based on the transmit signal which is subtracted by subtractor 121 from the receive signal in order to cancel any echo of the transmit signal that is present on the twisted wire pair that could interfere with the received signal. The residual echo signal is converted back to the frequency domain by fast fourier transform (FFT) in block 122.
The received signal, which now has had the cyclic prefix removed and has been converted back to the frequency domain is sent to block 124, where frequency domain equalization (FEQ) is employed to compensate for the channel distortion.
The received signal is then processed through a quadrature amplitude modulation (QAM) decoder 126 to decode the tone signal into digital data. That is followed by forward error correction (FEC) in block 128 which uses the syndrome bits that were added by the transmit path FEC encoder 106 to perform forward error correction. Finally, the data is descrambled in block 130 to extract the true data signal and then forwarded to a receiver 132.
The output of the FFT block 122 also is sent to a timing recovery circuit 136 that controls the CODEC 116 to synchronize the CODEC to the timing of the received data. Essentially, the timing recovery process is a feedback process in which the timing tones are detected and used to continuously adjust the CODEC timing so as to sample the received data at the appropriate sampling points.
In prior art frame alignment schemes, during initialization, the transmitter transmits a known pattern in a frame and the receiver attempts to receive that pattern. More particularly, the transmitter transmits the same pattern in a Who plurality of frames, for example 1024 consecutive frames. The receiver performs a cross-correlation of the data on the two wire pair with the expected pattern while the transmitter transmits those 1024 frames and determines which set of L+1 consecutive samples yields the peak cross correlation. The timing that resulted in the peak cross correlation calculation is selected as the start time of a frame or a symbol. It means that the sample that resulted in the peak cross correlation calculation is the first sample of the frame, e.g., 544 samples (before removing the cyclic prefix).
This technique for frame alignment or symbol alignment is computationally demanding and is not as accurate as desired.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for frame alignment in a DMT transceiver.
It is another object of the present invention to provide an improved DMT transceiver.